1. Field of the Invention
The present invention relates generally to communication systems and more particularly, to the assignment of Orthogonal codes in a code division multiple access communication system.
2. Description of Related Art
The Federal Communications Commission (FCC) governs the use of the radio frequency (RF) spectrum, deciding which industry gets certain frequencies. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many mobile stations as possible to have access to the spectrum.
Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads a transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required for transmitting the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wideband spreading code. Using this technique, a signal having the bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selected fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition. In addition, such a system has good performance in cases where interference may occupy a narrow-band.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously over the same frequency bandwidth. A particular receiver then determines which signal was intended for that receiver by a unique spreading code in the signal. The signals in that frequency bandwidth, without the particular spreading code intended for the particular receiver, appear to be noise to that receiver and are reduced by the processing gain of the system.
Because code division multiple access networks employ a system in which all transmissions occur in the same frequency band, it is well known that it is important to transmit at the lowest possible power that allows for the delivery of a communication signal at a certain level of accuracy or grade of service criteria. The reason that it is important for base stations to transmit to mobile stations with a minimal level of power and, on the reverse link, for mobile stations to transmit to base stations with minimal amount of power, is that each transmission adds to the noise level for all other receivers. In addition, if the per mobile station power on the forward link is minimized there is more power available for other mobile stations, thereby increasing the capacity of the system. Similarly on the reverse link, if less power is used, apart from the interference benefits mentioned above, the mobile station can extend its battery life and/or range of transmission.
In CDMA systems, the network consists of a plurality of cells. Each cell may also contain a plurality of sectors, depending on the deployment scenario. Each sector is distinguished from any of the other sectors by the use of a pseudo random code. In the IS2000/IS95 version of CDMA, these are known as PN sequences. In the UMTS version of CDMA, segments of Gold codes are used by each sector to accomplish the same effect. Therefore a user attempting to de-correlate signals from a particular sector must use the appropriate sequence. Within each sector a plurality of mobiles may be actively communicating with the system. Mobiles within the same sector are distinguished from one another by the use of Orthogonal codes. Therefore a particular user in a sector can extract its signal uniquely from the multitude of signals being transmitted by that sector. In IS2000/IS95 these Orthogonal codes are known as Walsh codes. In UMTS, these codes are generated by a technique call the OVSF or Orthogonal Variable Spreading Factor technique, but are essentially Walsh codes. For example, in IS2000 RC3 code division multiple access networks, there are up to 64 Walsh Codes that may be selected for use at 9600 bits/second to create a communication channel for users operating at this rate. In an IS2000 RC4 CDMA network, however, up to 128 Walsh Codes may be used for 9600 bits/second users. According to the system design, these Walsh Codes form the entire pool of codes that may be used either within a cell, or within a sector, according to the system design.
These Orthogonal codes, however, also must be used for overhead channels. Additionally, soft handoff reduces the number of Orthogonal codes that are available, as multiple codes have to be allocated for mobile stations that are in handoff. Accordingly, even though 64 or 128 Walsh Codes are available per sector or cell, in the above example, the use of Walsh Codes for the overhead and soft handoff conditions effectively limits the number of Walsh Codes that are available for assignment to mobile stations to approximately 30 or 60 Walsh Codes according to whether the system is an RC3 or RC4 system.
Stated differently, the limited number of Orthogonal codes limits the maximum number of simultaneous traffic channels that may be supported. In the past, the small number of Orthogonal codes had not been the limiting factor in terms of network capacity. Rather, the aforementioned power control issues have typically been the predominate factor in limiting access to a network. Smart antennas are now available, however, for use in code division multiple access communication systems.
Smart antennas increase the power-blocking limit by a large amount due to the intrinsic interference suppression properties that are associated with smart antennas. The power-blocking limit is a limit set by system designers at which to block incoming calls, to ensure that the Base Station power amplifier is not overdriven, and that an unstable system operating point threshold is not exceeded. As more users are allowed access to the system, power must be allocated on the forward link or downlink for each of the users. The Base station power amplifier can only handle a certain output power, before being damaged. In addition, if there were no limit to the output power, and assuming the Base station power amplifier does not burn out, a point will be reached where each user will start to rapidly require more and more power to meet the call quality requirements. Once this point is reached, each user individually could require large amounts of power to maintain their call quality. If the power were granted to these users, it would contribute large amounts of interference further fueling the power increase requirements. The point at which these unstable situation occurs is sometimes called asymptotic capacity. Thus, because smart antenna technology reduces the power required per sector for a mobile station by large amounts, it would appear that many more mobile stations can be supported in a sector or cell. The threshold, which is a function of the number of users, due to power blocking, has been significantly increased.
Because the power-blocking threshold has been increased, the Orthogonal code limitation discussed above becomes the limiting factor. Because Orthogonal code blocking, rather than power-blocking, will be the threshold limit to capacity of a telecommunication network in future systems employing smart antennas, the service provider can realize additional revenue if network capacity can be increased. However, because there are a limited number of Orthogonal codes in the system (as an example, only 64 Orthogonal Walsh Codes that may be used in an RC3 system and 128 Walsh Codes in an RC4 system), additional codes may not readily be created. It should be noted that the number of Orthogonal codes can be increased by lowering the data rate of each user in the sector, but for next generation services this is not a viable option. In addition, basic services such as voice require as certain operating data rate to ensure good call quality. Also, there is certain minimum operating rates specified in the IS2000 and UMTS standards.
As is known by those skilled in the art, the number of Orthogonal Codes that exists is limited by their very nature of being Orthogonal. Other approaches are being considered to increase the number of mobile stations that may be served in a defined service area. One approach is to define a group of quasi-Orthogonal codes. Quasi-Orthogonal codes can increase the number of usable codes by creating a family of code groups with certain properties. The codes used from the same family would be completely Orthogonal to each other, but the codes used from separate families would be somewhat Orthogonal, and as a result contribute somewhat less interference to each other. As a result, the use of quasi-Orthogonal codes only provides gain in fixed wireless access systems where there is no mobility or location change of the wireless terminals. In addition, only a limited number of quasi-Orthogonal codes can be used. This is because the large number of users using the regular Orthogonal codes from one family will provide very large interference to the few users using the quasi-Orthogonal codes from another family, and will cause them to transmit at very high powers. This may result in poor voice quality for some of the users, especially users nearer the cell edge where they would already be operating closer to their upper limits of power, and will ultimately reduce the aforementioned power blocking limit. Also, each group of quasi-Orthogonal functions requires their own pilot channel, for optimal performance, thereby using additional overhead power. For these and other reasons, the use of quasi-Orthogonal codes is problematic and reduces network reliability. Finally, it should be mentioned that the UMTS standard does not allow for the use of quasi-Orthogonal functions. At present, only the IS2000 standard allows their use.
What is needed, therefore, is a way of increasing the capacity of a code division multiple access network, and more particularly, increasing the number of communication channels notwithstanding the limitation of the number of Orthogonal codes that are available for use by a mobile terminal.